1. Field of the Invention
The invention relates to a control apparatus and method for a hybrid vehicle, and particularly to a control apparatus and method for a hybrid vehicle having an internal combustion engine and an electric motor as motive power sources.
2. Description of the Related Art
Hybrid vehicles that are driven by efficiently combining an internal combustion engine (engine) and an electric motor (motor) have been practically used in consideration of the environmental problems. As the engine installed in such hybrid vehicles, Japanese Patent Application Publication No. 2008-151064 (JP-A-2008-151064), for example, discloses the engine provided with an exhaust gas recirculation (to be referred to as Exhaust Gas Recirculation (EGR) hereinafter) device that recirculates some of the exhaust gas within an exhaust passage to an intake passage again.
This EGR device recirculates some of the exhaust gas discharged from the engine into the intake system, and reduces the combustion temperature by mixing the exhaust gas with new air-fuel mixture to thereby prevent the generation of nitrogen oxide (NOx) or to improve fuel efficiency by suppressing pumping loss.
From the perspective of further improvement of fuel efficiency, in the abovementioned engine provided with the EGR device, increasing the amount of exhaust gas (EGR gas) recirculated into the intake system, that is, increasing the EGR rate indicating a rate of the EGR gas amount with respect to the amount of intake air introduced into a cylinder, is taken into consideration. Increasing the EGR rate allows not only combustion to take place at a stoichiometric air-fuel ratio when the concentrations of the fuel and the oxygen are low, but also the combustion heat to be absorbed by the exhaust in the air-fuel mixture, so that increase in the amount of fuel consumption for adjusting the temperature of the exhaust gas can be suppressed.
On the other hand, in the EGR device, it is necessary to increase the diameter of an EGR pipe introducing the exhaust gas that has passed through a three-way catalytic converter into an EGR valve or to increase the size of the EGR valve, in order to realize high EGR rate. The problem in increasing the diameter of the EGR pipe or the size of the EGR valve is that the control responsiveness in controlling the opening of the EGR valve is reduced.
Specifically, in a case in which engine stop control is performed in response to a deceleration requirement from a driver, even when a valve closing signal is output to the EGR valve, a predetermined response time period is required until the EGR valve is actually completely closed, due to the impairment in the responsiveness of the EGR valve. For this reason, the EGR gas is continuously introduced into the intake pipe during the response time period. Furthermore, by performing valve closing control on a throttle valve, the amount of intake air is reduced and the negative pressure in the intake pipe rises, whereby the amount of recirculating EGR gas increases. In addition, the EGR gas recirculated into the inside of the intake pipe during the response time period remains in the intake pipe without being discharged into the combustion chamber by fresh air.
When the amount of the EGR gas left in the intake pipe increases, the amount of combustible air substantially decreases as the residual amount of the EGR gas increases. Thus the air-fuel ratio falls out of a combustible mixture ratio, thereby occurring a so-called misfire, which is a phenomenon in which the air-fuel mixture within the combustion chamber is not ignited. When a misfire occurs, the engine rotational speed decreases, deteriorating the drivability. Another problem is that there is a negative influence on an exhaust emission control catalyst when unburned air-fuel mixture is discharged to the exhaust passage.